Where a radar system (1) sits in a site where there is a known set of static ‘clutter’ targets (2) (FIG. 1) and is sending out a transmission signal (3), the phase of the return signal (4) from the static clutter targets should, all else being equal, be a constant. In practice, return signal readings vary in that there are changes in phase of the radar signal from the clutter targets. This variation is due to changes in the refractive index of the atmosphere through which the signal travels, which itself is dependent on temperature, pressure and humidity. The changes due to temperature and pressure are minor in comparison with the changes due to humidity, which means that if the phase return from known static clutter can be measured, it is possible to make a measurement of the humidity of the atmosphere. This measurement of humidity is of great interest in and can be used to improve weather measurement and prediction.
As is known in the art, weather radar typically has a frequency range in the GHz range. A magnetron based radar is preferable in terms of cost relative to other radar types, however magnetron based radars suffer from frequency variation such that whereas the production of a transmission signal of a particular frequency is intended, the actual signal produced commonly varies from this due to frequency changes caused by changes in the size of the magnetron cavity due to thermal expansion.
A GHz signal is at too high a frequency to process, so to overcome this it is standard practice to transfer or down convert the return signal to a lower frequency by mixing with a signal at a different frequency slightly below that of the transmitted signal. This mixing signal is generated using a Stable Local Oscillator (STALO). The resulting signal, which is typically in the MHz range, is known in the art as the Intermediate Frequency (IF).
Typically, a magnetron radar may have a base frequency of 5.625 GHz, which is mixed with a STALO signal at 5.595 GHz to produce an IF signal of 30 MHz. The base frequency, however, may vary from the 5.625 GHz by ±500 KHz due to the magnetron frequency variation.
Further, as well as slight variations in the transmitter signal, the STALO signal is also subject to minor variation. These combined variations in transmitter signal and STALO mixing signal mean that the IF is generally not sufficiently stable enough to provide the sensitivity to pick up the changes in phase of the radar signal which would be attributable to atmospheric humidity.